The year Erwin chose (23 B.C.) falls in the reign of Augustus, great-nephew of Julius Caesar and considered the first Roman emperor. His legions numbered nearly 330,000 men. They wore heavy leather and metal armor, carried swords and javelins, and operated catapults. They would have never heard the sound of an explosion before. “Obviously, there is a massive difference in firepower,” says Roman military expert and author Adrian Goldsworthy. “Not only would Roman armor be useless against a rifle round—let alone a grenade launcher or a .50 caliber machine gun—it would probably distort the bullet’s shape and make the wound worse.”

In the reddit story, however, Erwin said the Marines would not be resupplied with bullets, batteries, or gasoline from the modern world. “There would be no way of obtaining replacements for these supplies in the ancient world,” Goldsworthy says. “An average unit of Marines is not likely to be able to make an oil refinery, start generating electricity, or create machine tools to make spare parts for equipment.” And even if they could figure it out, it would take many months or even years. So, as soon as the Marines ran out of gas, their tanks would become little more than hunks of metal.

“In the short term and in the open, modern infantry could massacre any ancient soldiers at little risk to themselves,” Goldsworthy says. “But you could not support modern infantry. So all of these weapons and vehicles could make a brief, dramatic, and even devastating appearance, but would very quickly become useless. Probably in a matter of days.”